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Abstract:

The invention relates to a film composite and the use thereof as a mirror
film in solar reflectors in view of the sustainable guarantee of the
required reflection of solar radiation (total solar reflection). In
particular, the invention relates to the use of cover films on the basis
of polymethyl methacrylate (PMMA) in the film composite, having an
especially high UV stability, and a high weathering stability. The
invention further relates to a UV and weather protection package for said
solar mirror film, as utilized in solar reflectors, for improving the
optical life span, weathering stability, and for avoiding delamination.
The invention further relates to a surface finish with regard to scratch
resistance, anti-soil and chemical stability of the solar mirror film.

Claims:

1. A film composite for a solar reflector, comprising: an outer film
comprising a triazine, as a UV absorber, and a UV stabilizer; a backing
film; a metal layer; and a pressure-sensitive adhesive.

2. The film composite of claim 1, comprising, in sequence; an outer film
comprising a triazine, as a UV absorber, and a UV stabilizer; a backing
film; a metal layer; and a pressure-sensitive adhesive.

3. The film composite of claim 1, wherein the outer film is a PMMA-based
film, the backing film is a polyester film, a PMMA film, a two-layer
PVDF/PMMA film, or a PVDF/PMMA blend film, and the metal layer is a
silver layer or an aluminum layer.

4. The film composite of claim 1, wherein the backing film and the metal
layer are comprised in a metal-based layer.

5. The film composite of claim 1, further comprising: an adhesive layer
between the outer film and the backing film or between the outer film and
the metal layer.

6. The film composite of claim 1, wherein the outer film comprises: a
mixture of UV absorbers, comprising a triazine and a benzotriazole, and a
HALS compound as a UV stabilizer.

7. The film composite of claim 6, wherein the outer film comprises: from
0.1% by weight to 10% by weight of the benzotriazole, from 0.1% by weight
to 5% by weight of the triazine UV absorber, and from 0.1% by weight to
5% by weight of the HALS compound.

8. The film composite of claim 1, wherein the outer film comprises
poly(meth)acrylate and polyvinylidene fluoride in a ratio by weight of
from 1:0.01 to 0.3:1, and the outer film further comprises a UV
stabilizer, a UV absorber, or a mixture thereof.

9. The film composite of claim 8, wherein the outer film comprises: a
first sublayer comprising poly(meth)acrylate and a second sublayer
comprising polyvinylidene fluoride.

10. The film composite of claim 1, wherein the outer film is
scratch-resistant.

11. The film composite of claim 1, wherein a surface of the outer film
comprises an antisoiling coating.

12-13. (canceled)

14. A solar reflector with long service life, comprising the film
composite of claim 1, wherein a solar reflection decreases by at most 8%
over a period of 10 years.

15. The film composite of claim 3, wherein the metal layer is a silver
layer.

16. The film composite of claim 4, wherein the backing film and the metal
layer are comprised in an aluminum-based layer.

17. The film composite of claim 7, wherein the outer film comprises: from
0.5% by weight to 4% by weight of the benzotriazole, from 0.5% by weight
to 3% by weight, of the triazine, and from 0.2% by weight to 2% by
weight, of the HALS compound.

18. The film composite of claim 1, wherein the outer film comprises
poly(meth)acrylate and polyvinylidene fluoride in a ratio by weight of
from 1:0.1 to 0.4:1.

19. The film composite of claim 10, wherein the outer film comprises a
scratch-resistant coating.

20. A method of producing a solar reflector, comprising producing a solar
reflector with the film composite of claim 1.

21. The method of claim 20, wherein producing the solar reflector
comprises: applying the film composite, via the pressure-sensitive
adhesive, to a metal backing sheet, by film lamination.

22. The solar reflector of claim 14, wherein the solar reflection
decreases by at most 5% over a period of 10 years.

Description:

FIELD OF THE INVENTION

[0001] The invention relates to a film composite and to its use as
reflector film in solar reflectors, in the context of ensuring maximum
durability of the required reflection of solar radiation (total solar
reflection).

[0002] The invention in particular relates to the use of outer films based
on polymethyl (meth)acrylate (PMMA) in the film composite, with
particularly high UV resistance and high weathering resistance. The
invention further relates to a UV- and weathering-stabilizer package for
said solar reflector film, as used in solar reflectors, for improving
optical lifetime and weathering resistance, and for preventing
delamination. The invention further relates to a surface finish relevant
to scratch resistance, antisoil properties and chemicals resistance of
the solar reflector film.

[0003] The expression outer film is synonymous with a surface-finishing
layer, surface-finishing film, or the general term surface finish.

PRIOR ART

[0004] Flexible reflector film laminates in solar reflectors of the prior
art have disadvantages in respect of adequacy of weathering resistance,
and degradation due to ultraviolet (UV) radiation. However, in particular
for outdoor applications, stringent requirements are placed upon such
films relating to stability and resistance to UV radiation and to other
effects of weathering. These demands concern in particular dimensional
stability, reflectance in the visible and near-infrared spectrum, and
external appearance.

[0006] However, filtration should be used to remove the region below 400
nm, in particular below 375 nm, in order to lengthen the lifetime of the
solar reflector film, and the remaining "effective wavelength range" is
therefore from 375 nm or 400 nm up to 2500 nm.

[0007] The reflectance of flexible reflector film laminates in this
application is determined by quality and stability. This type of laminate
is produced by vacuum-metallization to give a thin silver layer on a
flexible polymer film (backing film). Silver is the preferred metal for
this application, because its reflectance in the relevant wavelength
range is particularly high in comparison with other metals. The polymeric
backing film and optionally the silver layer are protected from external
effects by an additional protective-layer coating. The purpose of this
protective layer, which is generally an outer surface-finishing film, is
to provide protection from mechanical abrasion, effects of weathering,
and degradation due to UV radiation.

[0008] U.S. Pat. No. 4,307,150 describes a protective system of this type
for aluminium reflectors. The backing film used comprises a PET laminate
which is metallized with aluminium and is protected from corrosion and
effects of weathering by using adhesive bonding to apply a (meth)acrylate
film. No explicit consideration is given to UV stabilizers. Because of
the higher reflectance, metallization with silver is a markedly more
suitable option than an aluminium layer in the design of these
reflectors. U.S. Pat. No. 4,645,714 describes the use of silver. However,
in principle silver has two disadvantages. Firstly, silver layers,
particularly if they are thin, are particularly susceptible to corrosion.
Protective layers or protective laminates therefore have to be
particularly leakproof. Otherwise, small perforations or inadequate leak
proofing at the edge of the laminate are sufficient to cause undesired
oxidation. Secondly, both silver and aluminium have an absorption gap in
the spectral region of ultraviolet light (from 300 to 400 nm), in
particular in the region around a wavelength of 320 nm, which is an
important constituent of sunlight. This permeability is particularly
present in very thin layers. This short-wave radiation is damaging to the
backing film and also to the reflector layer of metallic silver, and
particularly to the polyester films or polyester laminates that are
mostly used, or to the adhesives used for laminate production, especially
when exposure is prolonged, as is the case in the solar reflectors
sector. The result can be blistering and therefore deformation and
reduction of reflectance.

[0009] Inhibitors to counter corrosion, and UV-absorption reagents can be
introduced into the abovementioned protective film in order to improve
the level of protection. However, a disadvantage of most of the
inhibitors used is that they themselves have only relatively low
weathering resistance or indeed UV resistance, and cause discoloration
over the course of time. This discoloration reduces reflection in other
spectral regions and thus reduces the effectiveness of the solar
installation.

[0010] On the other hand, UV absorbers by themselves do not make a
contribution to prevention of corrosion of the silver coating: they
merely inhibit weathering-related ageing of the polyester laminate.
Optimized protection is therefore obtained by separating the two
components--inhibitor and UV absorber.

[0011] To that end, U.S. Pat. No. 4,645,714 applies two separate
(meth)acrylate-based coatings. The outer coating comprises the UV
absorber, and the inner coating comprises the inhibitor. By virtue of
this structure, the outer layer protects the inner layer, and the extent
of discoloration is markedly reduced. The inhibitor layer here is applied
directly to the silver metallization. The second layer, comprising the UV
absorber, is in turn applied over said first layer. The polyester
laminate used comprises a two-layer coextruded PET film, where one of the
layers comprises a lubricant to improve flexibility and the other,
silver-metallized, layer comprises no lubricant, in order to ensure
provision of a surface with maximum smoothness. The inner (meth)acrylate
layer comprises from 0.5 to 2.5% by weight of glycol dimercaptoacetate,
which acts as dispersing agent, coupling reagent, adhesion promoter and
inhibitor.

[0012] The outer (meth)acrylate layer comprises a UV-absorption reagent
which is active for radiation with a wavelength from 300 nm to 400 nm.
Corrosion of the silver by the UV-absorption reagent is moreover excluded
by virtue of the two-layer structure.

[0013] The uncoated side of the polyester laminate in turn has a coating
of a PSA (pressure-sensitive adhesive) based on an isooctyl
acrylate-acrylamide copolymer. This coating can, for example, be
protected with a silicone-coated polyester film prior to use of the
reflector laminate.

[0014] However, a disadvantage of both of the (meth)acrylate coatings
described in U.S. Pat. No. 4,645,714 and U.S. Pat. No. 4,307,150 is that
they have relatively low weathering resistance when used in outdoor
applications and therefore in principle have only poor suitability for
outdoor solar applications. These coatings have only very poor
water-repellency, and are not abrasion-resistant, and have only very
limited resistance to humidity. Once the poly(meth)acrylate coating has
been eroded, the polyester laminate can then be subject to the type of
degradation described. In order to avoid this problem, U.S. Pat. No.
5,118,540 uses adhesive bonding to apply an abrasion-resistant and
moisture-resistant film based on fluorocarbon polymers. Both the
UV-absorption reagent and the corrosion inhibitor are constituents of the
adhesive layer used to bond the film to the metal surface of the
metallized polyester backing film. The adhesive layer here can again, by
analogy with the double (meth)acrylate coating described above, be
composed of two different layers, in order to separate the corrosion
inhibitor from the UV-absorption reagent.

[0015] However, the UV-absorption reagents used are exclusively
benzotriazoles, which have only comparatively "brief" intrinsic stability
when exposed to UV radiation, and which--in said application--do not
provide effective UV protection for the adhesive layer itself and the
bonded metal surface and, respectively, polyester backing film.

[0016] In contrast, WO 2007/076282 lists an alternative structure to
improve protection of the silver coating. The PET backing film is now not
metallized with silver on the surface, but instead on the side facing
away from the light. On the other side of the PET film, adhesive bonding
is used to apply a protective poly(meth)acrylate film equipped with
UV-absorption reagents. The teaching relating to the requirement for
provision of durable UV protection is not considered in WO 2007/076282.

[0017] A pressure-sensitive adhesive (PSA) can be provided directly on the
reverse side of the silver metallization, or an additional copper layer
can be metallized onto the silver metallization in order to improve
corrosion resistance on the reverse side and in order to improve adhesion
of the PSA.

[0018] The poly(meth)acrylate film used for UV protection is a Korad®
film from SPARTECH PEP. A disadvantage of this film is that the UV
absorbers used comprise "benzotriazoles", which have only comparatively
"brief" intrinsic stability when exposed to UV radiation, and which--in
said application--do not provide effective UV protection for the adhesive
layer and the bonded polyester backing film.

[0019] Poly(meth)acrylate films which are similar to or analogous to the
Korad films have become established in use as surface-protection films
for plastics mouldings for various outdoor applications, e.g. in the
commercial vehicle sector. However, the performance of this type of outer
film is not adequate to provide the required long-term weathering
resistance in the reflector film application for solar reflectors,
involving maintenance of high solar reflectance.

[0020] The prior art provides no teaching concerning production of an
outer film with durable UV-protection function appropriate to the
durability requirements and reliability requirements placed upon solar
reflectors.

[0021] In the film laminates marketed hitherto for solar reflectors, the
only UV absorbers introduced into the outer film for stabilization with
respect to UV radiation are of benzotriazole type. These UV absorbers are
marketed by way of example with trade mark Tinuvin 234 by Ciba Specialty
Chemicals Inc. It is known that these UV absorbers undergo significant
losses of effectiveness over a period of from 5 to 10 years, the result
being degradation of the surface-finishing layer. This results in a
marked decrease in the solar reflection provided by the solar reflector
film composite.

[0022] There is an increasing requirement for solar reflector films which
markedly exceed the existing requirements placed upon the durability, or
maintenance of performance, of the established solar reflector films.

OBJECT

[0023] An object was to provide a novel solar reflector film composite for
solar reflectors with, in comparison with the prior art, improved or at
least equivalent optical properties, and improved weathering resistance,
in particular in respect of long-term use.

[0024] Long-term use here is in particular use over a period of more than
10 years, in particular more than 15 years, particularly preferably more
than 20 years.

[0025] A particular object was that the film composite for solar
reflectors remain stable for a long time when subject to particularly
strong insolation, as occurs by way of example in the Sahara or in the
south west of the USA. This particularly affects the intrinsic stability
and filter efficiency of the outer film of said composite films with
respect to the UV wavelength spectrum from 300 nm to 400 nm.

[0026] Another object was to provide a film composite for solar
reflectors, where the production of, and downstream operations on, these
are simple and cost-efficient.

[0027] A further intention is that the increased-stability film composite
have minimum colour and maximum resistance to moisture.

[0028] A further intention is that the film composite be scratch-resistant
and have dirt-repellent properties.

ACHIEVEMENT OF OBJECT

[0029] In the light of the prior art and the shortcomings of the technical
solutions described therein for long-term applications, the present
invention is successful in a manner which was not readily foreseeable by
the person skilled in the art in providing a film composite which is
transparent except for the metal layer and which has improved weathering
resistance and provides good and stable solar reflection over a long
period, and which also has a number of further advantages.

[0030] The object is achieved via provision of a novel film composite
which can be used in solar reflectors. Said film composite is composed of
at least the following layers: an outer film, a backing film, a metal
layer, and a pressure-sensitive-adhesive layer. In particular, this is a
film composite in which the outer film is a PMMA-based film, the backing
film is a polyester film, and the metal layer is a silver layer. Below
the outer film there is also optionally an adhesive layer applied, and
below the metal layer there is also optionally a migration-barrier layer
and/or a primer applied.

[0031] Said film composite is preferably composed of at least the
following layers, in the sequence listed, from the subsequent external
side to the PSA layer, which is bonded to a backing material: an outer
film, a backing film, a metal layer, and a pressure-sensitive-adhesive
layer. Between the outer film and the backing film there is also
optionally an adhesive layer applied, and between the metal layer and the
pressure-sensitive adhesive there is also optionally a migration-barrier
layer and/or a primer applied.

[0032] Another important feature of the present invention is that the
novel film composite features very high stability, in particular UV
resistance, even when subject to prolonged irradiation. To this end, the
outer film comprises at least one triazine as UV absorber, and one UV
stabilizer. In particular, a mixture of UV absorbers is involved,
composed of at least one triazine and of at least one benzotriazole. The
UV stabilizer is a HALS compound or a mixture of various HALS compounds.
Triazine-based UV absorbers exhibit particularly high intrinsic stability
when exposed to UV radiation.

[0033] Stability is the intrinsic stability of the outer film with respect
to UV effects and weathering effects, and at the same time the stability
of the UV-protection effect, discernible by way of example from
maintenance of solar reflection.

[0034] The outer film can preferably comprise from 0.1% to 10% by weight,
preferably from 0.2% by weight to 6% by weight, and particularly
preferably from 0.5% by weight to 4% by weight, of the benzotriazole-type
UV absorbers,

[0035] from 0.1% to 5% by weight, preferably from 0.2% to 3% by weight,
and particularly preferably from 0.5% by weight to 3% by weight, of the
triazine-type UV absorbers, and

[0036] from 0.1% by weight to 5% by weight, preferably from 0.5% by weight
to 3% by weight, and particularly preferably from 0.2% by weight to 2% by
weight, of the UV stabilizers, preferably HALS-type UV stabilizers.

[0037] The mixture used according to the invention and composed of UV
absorbers and of UV stabilizers exhibits stable, durable UV protection
over a wide wavelength spectrum (from 300 nm to 400 nm).

[0038] The outer film, comprising the mixture composed of UV stabilizers
and of UV absorbers, can optionally be composed of poly(meth)acrylate and
polyvinylidene fluoride in a ratio by weight of from 1:0.01 to 1:1,
preferably from 1:0.1 to 1:0.5. It is preferable that the outer film
encompasses two sublayers. One sublayer here is a sublayer composed of
poly(meth)acrylate and the other is a sublayer composed of polyvinylidene
fluoride (PVDF). It is preferable that the PVDF sublayer is the layer
located at the surface of the film composite.

[0039] Irrespective of the composition, the thickness of the outer film is
in the range from 10 μm to 200 μm, preferably in the range from 40
μm to 120 μm, particularly preferably in the range from 50 μm to
90 μm.

[0040] The outer film used according to the invention has optionally been
rendered scratch-resistant, preferably by virtue of a scratch-resistant
coating. The surface of the outer film can moreover have been equipped
with an antisoiling coating.

[0041] The novel film composite according to the invention moreover has a
combination of the following properties as advantage over the prior art,
particularly in respect of optical properties: the transparent part of
the film composite according to the invention has particularly little
colour and does not become cloudy when exposed to moisture. The film
composite also exhibits excellent weathering resistance and, if equipped
optionally with a PVDF surface and/or if rendered scratch-resistant, has
very good chemicals resistance, for example with respect to any
commercially available cleaning composition. These are further aspects
contributing to maintenance of solar reflection over a long period.

[0042] To facilitate cleaning, the surface has dirt-repellent properties.
The surface is also optionally abrasion-resistant and/or
scratch-resistant.

[0043] The film composite according to the invention is used in particular
as reflector film in solar reflectors. The reflector film is applied to a
metal backing sheet, for example an aluminium sheet, by means of film
lamination, and the adhesion to the metal backing sheet is brought about
by the pressure-sensitive-adhesive layer.

[0044] The film composite according to the invention in particular
features UV resistance markedly improved over the prior art, and the
longer lifetime associated therewith. The material according to the
invention can therefore be used in solar reflectors over a very long
period of at least 10 years, indeed preferably at least 15 years, and
particularly preferably at least 20 years, at locations with a
particularly large number of hours of sunlight and with particularly
intensive insolation, examples being the south-west of the USA and the
Sahara.

[0045] The maximum decrease in solar reflection of the durable solar
reflector equipped with the film composite according to the invention
over a period of 10 years is 8%, preferably 5%, and particularly
preferably 3%.

DETAILED DESCRIPTION OF THE INVENTION

[0046] The outer films ideally used for UV protection of the reflector
film composites structured according to the invention correspond to the
UV-protective films disclosed in WO 2007/073952 (Evonik Rohm) or DE 10
2007 029 263 A1. These films can, by way of example, have the
constituents briefly outlined hereinafter. A more comprehensive
description is found in WO 2007/073952. The outer films used according to
the invention are PMMA-based films. The wording PMMA-based films does
not, however, restrict the films to straight methacrylate compositions or
to a single-layer structure. Instead of this, the PMMA in the films can
comprise comonomers which are not methacrylates. The films can also be
composed of blends of various plastics, and it is not necessary that all
of these contain methacrylates. The films can also comprise a polybutyl
acrylate elastomer fraction for impact-modification. The outer film can
moreover be composed of more than two layers. It is not essential that
all of these layers comprise methacrylates.

Production of the PMMA Plastics

[0047] Polymethyl methacrylate plastics are generally obtained by
free-radical polymerization of mixtures which comprise methyl
methacrylate. These mixtures generally comprise at least 40% by weight,
preferably at least 60% by weight and particularly preferably at least
80% by weight, based on the weight of the monomers, of methyl
methacrylate.

[0048] These mixtures for production of polymethyl methacrylates can also
comprise other (meth)acrylates copolymerizable with methyl methacrylate.
The expression (meth)acrylates comprises methacrylates and acrylates and
mixtures of the two. These monomers are well known.

[0049] The compositions to be polymerized can also comprise, as well as
the (meth)acrylates described above, other unsaturated monomers which are
copolymerizable with methyl methacrylate and with the abovementioned
(meth)acrylates. Among these are, inter alia, 1-alkenes, such as
1-hexene, acrylonitrile; vinyl esters, such as vinyl acetate; styrene or
a-methylstyrene. The amount generally used of these comonomers is from 0%
by weight to 60% by weight, preferably from 0% by weight to 40% by
weight, and particularly preferably from 0% by weight to 20% by weight,
based on the weight of the monomers, and these compounds can be used
individually or in the form of a mixture.

[0050] The outer films used according to the invention also have major
advantages in production. The components used, e.g. the UV stabilizers
and the UV absorbers, permit economic operation of an extrusion plant. By
way of example, no gases are evolved during the film extrusion process.
There is therefore no need for complicated purging processes that are
detrimental to quality.

Impact-Modified poly(meth)acrylate

[0051] The PMMA-based film used according to the invention can comprise
impact modifiers. A more detailed description of these impact modifiers
is also found in WO 2007/073952.

PMMA/PVDF Films

[0052] In one particular embodiment of the invention, it is also possible
to use PMMA/PVDF films as outer film instead of straight methacrylate
films. PVDF (polyvinylidene fluoride) has some advantages as constituent
of a polymer blend or as laminate: PVDF has high chemicals resistance and
low surface energy. PVDF is therefore water-repellent and is comparable
with biocidal materials in that, even in long-term use, it is highly
resistant to colonization by organisms.

[0053] It is also possible to achieve high transparency in very thin PVDF
layers of thickness from 1 μm to 10 μm, preferably from 2 μm to
5 μm. A detailed description of the production of PMMA/PVDF films is
found in DE 10 2007 029 263 A1.

[0054] The PMMA/PVDF films can be produced in the form of monofilms
(produced by chill-roll processes) or in the form of films having more
than one sublayer (produced by means of lamination of two films or
coextrusion of the corresponding layers of melt), and both variants here
can achieve all of the advantages mentioned for the product. The UV
stabilizers and/or UV absorbers can be present here in one or both of the
films.

[0055] The ratio of poly(meth)acrylate to polyvinylidene fluoride, both in
monofilms and in films having more than one sublayer, is in the range
from 1:0.01 to 0.3:1 (based on weight). Even more preference is given to
modifications in which the film encompasses a mixture of
poly(meth)acrylate and polyvinylidene fluoride in a ratio of from 1:0.1
to 0.4:1.

[0056] The PVDF polymers used for the purposes of the invention are
polyvinylidene fluorides, i.e. generally transparent, semicrystalline,
thermoplastic fluoroplastics. The fundamental unit for polyvinylidene
fluoride is vinylidene fluoride, which is reacted (polymerized) in
ultra-pure water under controlled conditions of temperature and pressure
by means of a specific catalyst to give polyvinylidene fluoride.
Vinylidene fluoride is in turn available by way of example from hydrogen
fluoride and methylchloroform as starting materials, by way of
chlorodifluoroethane as precursor. For the purposes of the invention,
very successful results can be obtained in principle by using any of the
PVDF grades marketed. Among these are, inter alia, Kynar® grades
produced by Arkema, Dyneon® grades produced by Dyneon and Solef®
grades produced by Solvay.

The Stabilizer Package (Light Stabilizer)

[0057] A particular constituent of the UV-protective films used according
to the invention is the UV-stabilizer package, and this will be described
in more detail below.

[0058] Light stabilizers are well known, and are described in detail by
way of example in Hans Zweifel, Plastics Additives Handbook, Hanser
Verlag, 5th Edition, 2001, pp. 141 ff. Light stabilizers are UV
absorbers, UV stabilizers and free-radical scavengers. It is therefore
possible to select UV absorbers by way of example from the group of the
substituted benzophenones, salicylates, cinnamates, oxanilides,
benzoxazinones, hydroxyphenylbenzotriazoles, triazines, and
benzylidenemalonate.

[0059] The best known representative of the UV stabilizers/free-radical
scavengers is the sterically hindered amines group (hindered amine light
stabilizer; HALS).

[0060] The stabilizer package used in the films used according to the
invention is composed of the following components: [0061] Component A:
a benzotriazole-type UV absorber, [0062] Component B: a triazine-type UV
absorber, [0063] Component C: a UV stabilizer, preferably a HALS
compound.

[0064] Components A and B can be used as a single substance or in a
mixture. At least one UV-absorber component must be present in the film.
Component C is essential in the film used according to the invention.

Component A: Benzotriazole-Type UV Absorber

[0065] Examples of UV absorbers of benzotriazole type that can be used are
2-[2-hydroxy-5-methylphenyl)benzotriazole,
242-hydroxy-3,5-di(alpha,alpha-dimethylbenzyl)phenyl]benzotriazole,
2-(2-hydroxy-3,5-di-tert-butyl-phenyl)benzotriazole,
2-(2-hydroxy-3,5-butyl-5-methylphenyl)-5-chloro-benzotriazole,
2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole,
2-(2-hydroxy-3,5-di-tert-amylphenyl)benzotriazole,
2-(2-hydroxy-5-tert-butyl-phenyl)benzotriazole,
2-(2-hydroxy-3-sec-butyl-5-tert-butylphenyl)benzotriazole and
2-(2-hydroxy-5-tert-octylphenyl)benzotriazole, phenol,
2,2'-methylenebis[6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl)].

[0066] The amounts used of the UV absorbers of benzotriazole type are from
0.1% by weight to 10% by weight, preferably from 0.2% by weight to 6% by
weight and very particularly preferably from 0.5% by weight to 4% by
weight, based on the weight of the monomers used to prepare the
polymethyl methacrylates. It is also possible to use mixtures of
different UV absorbers of benzotriazole type.

Component B: Triazine-Type UV Absorber

[0067] Triazines, such as
2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol, can moreover also
be used as UV stabilizers in the mixture.

[0068] The amounts used of the triazines are from 0.0% by weight to 5% by
weight, preferably from 0.2% by weight to 3% by weight and very
particularly preferably from 0.5% by weight to 2% by weight, based on the
weight of the monomers used to prepare the polymethyl methacrylates. It
is also possible to use mixtures of different triazines.

Component C: UV Stabilizers

[0069] An example which may be mentioned here for free-radical
scavengers/UV stabilizers is sterically hindered amines, known as HALS
(Hindered Amine Light Stabilizer). They can be used to inhibit ageing
phenomena in paints and plastics, especially in polyolefin plastics
(Kunststoffe, 74 (1984) 10, pp. 620-623; Farbe+Lack, Volume 96, 9/1990,
pp. 689-693). The tetramethylpiperidine group present in the HALS
compounds is responsible for the stabilizing effect. This class of
compound can have no substitution on the piperidine nitrogen or else
substitution by alkyl or acyl groups on the piperidine nitrogen. The
sterically hindered amines do not absorb in the UV region. They scavenge
free radicals that have been formed, whereas the UV absorbers cannot do
this. Examples of HALS compounds which have stabilizing effect and which
can also be used in the form of mixtures are:
bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate,
8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspiro(4,5)-decane-2,5--
dione, bis(2,2,6,6-tetramethyl-4-piperidyl) succinate,
poly(N-β-hydroxyethyl-2,2,6,6-tetramethyl-4-hydroxypiperidine
succinate) or bis(N-methyl-2,2,6,6-tetramethyl-4-piperidyl) sebacate.

[0070] The amounts used of the HALS compounds are from 0.0% by weight to
5% by weight, preferably from 0.1% by weight to 3% by weight and very
particularly preferably from 0.2% by weight to 2% by weight, based on the
weight of the monomers used to prepare the polymethyl (meth)acrylates. It
is also possible to use mixtures of different HALS compounds.

[0071] Other costabilizers that can be used moreover are the HALS
compounds described above, disulphites, such as sodium disulphite, and
sterically hindered phenols and phosphites.

The Adhesive Layer

[0072] As a function of the production process, the optional adhesive
layer serves for bonding of the outer film to the backing film. It is
present only when direct adhesion between the two films is not possible.
The selection of the appropriate adhesive depends on the composition of
the two films--for example PMMA and PET--and on optical properties. The
adhesive layer, too, must have high transparency. Specific acrylate
adhesives can be suitable, for example.

The Scratch-Resistant Coating

[0073] The expression scratch-resistant coating in the context of this
invention is a collective term for coatings which are applied in order to
reduce surface scratching and/or to improve abrasion resistance. High
abrasion resistance is of particularly high importance for the use,
according to the invention, of the film composites in solar reflectors.

[0074] Another important property of the scratch-resistant coating in the
widest sense is that said layer does not adversely affect the optical
properties of the film composite.

[0075] The scratch-resistant coating used can comprise polysiloxanes, such
as CRYSTALCOAT®MP-100 from SCD Technologies Inc., AS 400-SHP 401, or
UVHC3000K, both from Momentive Performance Materials. These coating
formulations are applied by way of example by way of roller coating,
knife coating, or flow coating, to the surface of the film composite or
of the outer film.

[0076] Examples that may be mentioned of other coating technologies that
can be used are PVD plasma (physical vapour deposition; physical
gas-phase deposition) and CVD plasma (chemical vapour deposition;
chemical gas-phase deposition).

The Backing Film

[0077] The selection of the backing film is determined by the following
essential properties: the film must be a high-transparency, flexible,
heat-resistant film that can be metallized with a thin metal layer. To
this end, the metal layer should exhibit no loss of adhesion over a long
period. Films that have proved to have this type of property profile are
in particular polyester films, and very particularly coextruded,
biaxially oriented polyethylene terephthalate films (PET). These have
optionally been equipped with adhesion promoters to improve adhesion of
the metal layer, surface-finishing layer, or, respectively, adhesive
layer. The backing films used can also alternatively comprise PMMA films,
two-layer PVDF/PMMA films, or films composed of PVDF/PMMA blends.

The Metal Layer

[0078] The metal layer is preferably applied to the reverse side of the
backing film and is preferably composed of silver or aluminium,
particularly preferably of silver. On the side facing away from the
backing film, the metal layer can optionally be covered with a second
metal layer, for example composed of copper or of a nickel-chromium
alloy. This then serves firstly to protect the metal reflector layer and
secondly to improve the adhesion of the pressure-sensitive-adhesive
layer. As an alternative to silver, aluminium is used as metal for the
reflector layers. It is preferable here that the layers known as
"enhancement stack" layers are applied (by means of physical vapour
deposition) to raise the comparatively "low" reflection level of the
aluminium reflector in the relevant wavelength range.

[0079] As an alternative to use in the film composite described, the outer
films used according to the invention can also be used for surface finish
or, respectively, to improve the weathering-protection provided to
multilayer aluminium-strip-based systems such as those marketed as
MIRO-SUN® by Alanod solar.

Particular Embodiment

[0080] In one particular embodiment, a metal-layer system is used in the
form of a metal strip, preferably based on aluminium, an example being
MIRO-SUN®; this serves simultaneously as backing film. The backing
film and the metal layer are therefore identical in this embodiment, and
there is no need for any further polymer-based backing film.

The Pressure-Sensitive-Adhesive Layer

[0081] The pressure-sensitive-adhesive layer serves for the adhesive
bonding of the film composite for example to a backing material, e.g. a
curved aluminium sheet. The selection of the pressure-sensitive adhesive
(PSA) is determined via the adhesion with respect to said backing
material and with respect to the reverse side of the metal coating. For
protection of the metal coating it would also be advantageous that the
pressure-sensitive-adhesive layer has only low permeability to
atmospheric oxygen and water vapour.

[0082] For the purposes of transport and storage, the
pressure-sensitive-adhesive layer is applied to a siliconized paper. It
can in turn easily be removed prior to adhesive bonding to the backing
material.

Migration-Barrier Layer and Primer

[0083] Materials used as migration barrier inhibit migration of
constituents damaging to the metal layer, e.g. atmospheric oxygen, water
vapour, or else constituents that can migrate from the pressure-sensitive
adhesive. By way of example, this can be an epoxy-resin layer.

[0084] The selection of, and the need for, a primer is determined by the
adhesion properties or surface properties of the metal layer and of the
pressure-sensitive adhesive used.

Alternative Film-Composite Structure

[0085] As an alternative to the layer sequence described above for the
film composite systems according to the invention, composed--from the
outside to the inside--of an optional scratch-resistant coating, an
optional dirt-repellent coating, an outer film, an optional adhesive
layer, a backing film, the metal layer, an optional primer and/or
migration-barrier layer, and a pressure-sensitive adhesive, the film
composite can also be composed of the following, again in the following
layer sequence--from the outside to the inside: [0086]
scratch-resistant coating and/or dirt-repellent coating (optional) [0087]
outer film [0088] adhesive layer (optional) [0089] metal layer [0090]
primer and/or migration barrier [0091] backing film [0092]
pressure-sensitive adhesive

Production of Film Composite

[0093] As a function of intended use, the outer film used according to the
invention in the form of monofilm or film having more than one sublayer
can be produced with any desired thickness. A decisive factor here is
always the high transparency of the outer film, coupled with exceptional
weathering resistance, and also with the extremely high level of
weathering protection provided to the substrate.

[0094] The single- or multilayer outer film is produced via methods known
per se, an example being extrusion through a flat-film die, blown-film
extrusion, or solution casting.

[0095] Examples of methods for producing the film composite are lamination
and/or (co)extrusion coating.